In recent years, an exponential data traffic and electrical power consumption growth is exhibited in high performance computing (HPC) interconnects. Thus, Green Photonics, which tries to minimize the Joule/bit ratio becomes a major subject of both public and scientific interest. A significant amount of the installations within these HPCs are graded-index multimode fibers (GI-MMFs) using parallel rack-to-rack optical interconnects. These fibers enable the use of mode division multiplexing to increase the fiber's capacity.
High speed transmission systems (>10 Gb/s) for short-reach interconnects, such as data-centers and high performance computing (HPC), attracts extensive interest due to the exponential data traffic growth in such applications. The data traffic growth is due to an increase demand for cloud computing, video streaming, and proliferation of smart devices.
On the other hand, this also means that the power consumed by data centers continues to grow exponentially and the fiber volume becomes a significant challenge. Keeping limited natural resources in mind, the topic of Green Photonics has become in the focus of public and scientific interest.
Especially, as copper-based interconnect technology is inefficient, expensive and slow, the transition to optical interconnects has become a reality, with most of the power being consumed by sending data via interconnects within and between racks of servers.
In order to increase the fiber density in HPC systems, parallel optical interconnects are extensively deployed for rack-to-rack interconnects using commercial fiber-coupled optical modules and active optical cables with 8-12 fibers operating at data rages up to 10 Gb/s. Typical commercial Datacom optical modules utilize arrays of Vertical Cavity Surface Emitting Lasers (VCSELs) and GaAs PIN photodiodes coupled to standard 50-μm core multi-mode fiber designed for interconnect distances up to 300 m at 10 Gb/s.
Prior art documents suggested to increase the fiber bandwidth density by the use of multicore fibers (MCFs) with a GI-MMF core using a two-dimensional (2D) array of 850 nm VCSEL (B. G. Lee, D. M. Kuchta, F. E. Doany, C. L. Schow, P. Pepeljugoski, C. Baks, T. F. Taunay, B. Zhu, M. F. Yan, G. E. Oulundsen, D. S. Vaidya, W. Luo, and N. Li, “End-to-end multicore multimode fiber optic link operating up to 120 Gb/s,” J. Lightw. Technol., vol. 30, no. 6, pp. 886-892, March 2012). Or alternatively reduce manufacturing and maintenance cost by redundant VCSELs in each GI-MMF in a space-parallel fiber transmission scheme (H. Roscher, F. Rinaldi, and R. Michalzik, “Small-pitch flip-chip-bonded VCSEL arrays enabling transmitter redundancy and monitoring in 2-D 10-Gbit/s space-parallel fiber transmission,” IEEE J. Sel. Topics Quantum Electron., vol. 13, no. 5, p. 1279-1289, September/October 2007).
These recent abilities, open a new opportunity for spatial multiplexing optical multiple-input-multiple-output (MIMO) schemes in short reach interconnects, as different mode groups in the GI-MMF are acting as independent communication channels, i.e., mode-group-division-multiplexing (MGDM).
In prior art documents, the performance of intensity modulated spatial multiplexing multiple-input-multiple-output (MIMO) systems over optical fiber channels has been mainly verified by computer simulations and/or experimental results (G. Stepniak, L. Maksymiuk, and J. Siuzdak, “Influence of mode coupling on mode group diversity multiplexing in multimode fibers,” Opt. Quant. Electron., vol. 41, pp. 203-213, 2009, C. P. Tsekrekos, A. Martinez, F. M. Huijskens, and A. M. J. Koonen, “Design considerations for a transparent mode group diversity multiplexing link,” IEEE Photon. Technol. Lett, vol. 18, no. 22, pp. 2359-2361, November 2006, and M. Kowalczyk, and J. Siuzdak, “Four-channel incoherent MIMO transmission over 4.4-km MM fiber,” Microw. Opt. Technol. Lett, vol. 53, no. 3, pp. 502-506, March 2011), However, and to the best of the authors' knowledge, these previous works did not look into analytical approaches to optimally allocate system parameters, such as electrical power, and DC offset of the modulated signal. It should be noticed that the problem the present invention deals with is different from the ones used for the RF MIMO systems since the transmitted signal is constrained to be nonnegative, while RF MIMO solved this issue with no such constraint.
It is therefore an object of the present invention to optimize power allocation for each optical transmitter in an optical transmission system which consists of intensity modulated optical transmitter, an optical channel that can be spatially multiplexed and direct detection optical detectors.
It is another object of the present invention to provide a solution to the margin adaptive problem.
Other objects and advantages of the invention will become apparent as the description proceeds.